The present invention relates to the electrolytic production of high purity alkali metal hydroxide solutions. The alkali metal hydroxides of the present invention are produced along with halides utilizing membrane electrolytic cells by the passage of an electric current through a alkali metal halide solution.
Electrolytic cells that are commonly employed commercially for the conversion of alkali metal halides into alkali metal hydroxides and halides may be considered to fall into the following general types: (1) diaphragm, (2) mercury, and (3) membrane cells.
Diaphragm cells utilize one or more diaphragms permeable to the flow of electrolyte solution but impervious to the flow of gas bubbles. The diaphragm separates the cell into two or more compartments. Upon imposition of a decomposing current, halide gas is given off at the anode, and hydrogen gas and alkali metal hydroxide are formed at the cathode. Although the diaphragm cell achieves relatively high production per unit floor space, at low energy requirements and at generally high current efficiency, the alkali metal hydroxide product, or cell liquor, from the catholyte compartment is both dilute and impure. The product may typically contain about 12 percent by weight of alkali metal hydroxide along with about 12 percent by weight of the original, unreacted, alkali metal chloride. In order to obtain a commercial or salable product, the cell liquor must be concentrated and purified. Generally, this is accomplished by evaporation. Typically, the product from the evaporators is about 50 percent by weight alkali metal hydroxide containing about 1 percent by weight alkali metal chloride.
Mercury cells typically utilize a moving or flowing bed of mercury as the cathode and produce an alkali metal amalgam on the mercury cathode. Halide gas is produced at the anode. The amalgam is withdrawn from the cell and treated with water to produce a high purity alkali metal hydroxide. Although mercury cell installations have a high initial capital investment, undesirable ratio of floor space per unit of product, relatively poor efficiencies, and negative ecological considerations, the purity of the alkali metal hydroxide product is an inducement to its use. Typically, the alkali metal hydroxide product contains less than 0.05 percent by weight of contaminating foreign anions.
Membrane cells utilize or more membranes or barriers separating the catholyte and the anolyte compartments. The membranes are permselective, that is, they are selectively permeable to either anions or cations. Generally, the permselective membranes utilized are cationically permselective. In membrane cell employing a single membrane, the membrane may be porous or non-porous. In membrane cells employing two or more membranes, porous barriers are generally utilized closest to the anode, and non-porous membranes are generally utilized closest to the cathode. The catholytic product of the membrane cell is a relatively high purity alkali metal hydroxide, usually about 250 to about 350 grams per liter, with about 0.5 percent by weight foreign anions. Examples of membrane cells are described in U.S. Pats. Nos. 3,017,338; 3,135,673; 3,222,267; 3,496,077; 3,654,104; 3,899,403; 3,954,579; and 3,959,085. The catholytic product, or cell liquor, from a membrane cell is purer and of a higher concentration than the product of a diaphragm cell. One of the disadvantages of membrane cells is the relatively short and unpredictable longevity of the membrane material when exposed to the operating conditions encountered in commercial operations.
It has been the objective, but not the result, for diaphragm and membrane cells to commercially produce "rayon grade" alkali metal hydroxide, that is, a product having a contamination of less than about 0.5 percent of the original salt. Diaphragm cells have not been able to produce such a product directly, because anions of the original salt freely migrate into the catholyte compartment of the cell. Membrane cells have the capability to produce a high purity alkali metal hydroxide product but have not been able to consistently and continuously produce such a product because of membrane malfunction, especially of the more efficient hydraulically impermeable types utilized closest to the cathode.
A particularly useful arrangement of membranes in a three compartment cell utilizing a buffer compartment and used to electrolyze and alkali metal halide solution is to position a permeable barrier between the anolyte and the buffer compartments and a hydraulically impermeable cation-permeable membrane between the catholyte and the buffer compartments. The arrangement permits the flow of liquid to and from the anolyte compartment while inhibiting the flow of halogens outward from the anolyte compartment. However, because the porous barrier is not a perfect and absolute barrier to halogens and hypohalites, some of these materials migrate into the buffer compartment to the detriment of the hydrocarbon ion exchange membrane separating the catholyte compartment. The barrier between the catholyte compartment and the adjacent buffer compartment is hydraulically impermeable to solutions, but is selectively permeable to cations, thus allowing alkali metal ions from the buffer compartment to pass therethrough the react with the hydroxyl ions formed in the catholytic compartment. Various arrangements of membranes and various types of membrane materials have been proposed. The present invention is useful in membrane cells and is not limited to any specific compartment arrangement of type of membrane, except that the present invention is particularly adapted to membrane cells having at least one buffer compartment and to membrane cells which include a hydrocarbon membrane.